1. Field of the Invention
The invention relates to a method and apparatus for data recovery, and more particularly, to a method and apparatus for data recovery suitable for high-density data recording applications, that can effectively lower the data error rate.
2. Description of the Related Art
When transmitting data and when recording data on a recording medium, such as a floppy disk, an optical disc or a magneto-optic disc, data are modulated into another encoded data format based on a specific encoding rule for suitability to transmission or recording. In order to correctly receive and retrieve the encoded data from the recording medium, the length of the encoded data must conform with a run length limit (abbreviated as RLL). Taking the DVD system as an example, the run length of the RLL encoded data between two neighboring symbols conforms with (d=2, k=10) encoding constraint, where (d) is the minimum run length and (k) is the maximum run length. That is to say, in NRZI (non-return zero inverse) modulation, the number of ‘0’s between two neighboring ‘1’s must be at least (d) but must not exceed (k). On the other hand, in NRZ (non-return zero) modulation, the number of ‘0’s or ‘1’s must be at least (d′=d+1) but must not exceed (k′=k+1). Therefore, the encoded data can be correctly received and decoded. Due to continuous progress in recording techniques, it is foreseeable to have higher density recording in optical discs. For high frequency signals, the amplitudes of the signals however will decrease, and therefore result in lower signal-to-noise ratios (SNR). As a result, encoded data with run length equal to (d) or (d′) and (k) or (k′) will be affected by low frequency noise, causing incorrect decoding during data retrieving. Referring to FIGS. 1(a) to 1(c), with no consideration of any noise, assuming that an input signal (EQRF) employing RLL (d=1, k=7) in NRZ modulation, if the input signal (EQRF) is decoded based on a reference signal level (SL1), a digital signal (SLRF1) conforming with the run length encoding constraint (d′=d+1) will be generated. As shown in FIGS. 2(a) to 2(d), with noise (NOISE) interference, the amplitude of the input signal (EQRF) however will vary. Thus, an erroneous digital signal (SLRF2), which does not conform with the run length encoding constraint (d′=d+1) and which cannot be correctly recovered, will be generated.
In order to resolve the aforesaid issue, a data detection and correction method disclosed in U.S. Pat. No. 6,111,833, entitled “Data Decoder,” use patterns with run lengths of (d′−1), (k′−1), (d−1), (k′+1), (k+1), (d′−2), (d−2), (k′+2), and (k+2) to detect and correct erroneous data bits. Nevertheless, a high-resolution but higher cost analog-to-digital converter is required. Furthermore, no bit errors within a run length (d′−1) can be detected.